A lithium ion secondary battery has characteristic high energy density and electromotive power as compared to a lead storage battery or a nickel-hydrogen battery; therefore, a lithium ion battery has been widely used as a power supply for various portable devices which are demanded to be smaller in size and lighter in weight, and laptop computers. Such a lithium ion secondary battery is generally produced by a method in which a cathode sheet formed of a cathode current collector coated with a cathode active material and an anode sheet formed of an anode current collector coated with an anode active material are laminated through a separator and an electrolyte which are provided therebetween, and the resultant laminate of the cathode sheet, the separator and the anode sheet is sealed within a casing together with the electrolyte. The electrolyte may be a gel electrolyte as well as a liquid or solid electrolyte.
The lithium ion secondary batteries are available in the form of a cylinder, a can or a laminated pack, and, recently, a sheet lithium ion secondary battery having a laminated structure is also proposed.
In the case of a lithium ion secondary battery in the form of a sheet, when the sheet battery is bent to curve, a displacement occurs due to winding between the cathode, the separator and the anode which are laminated one upon the other. Here, as shown in FIG. 5, a plurality of bent portions 101 occur on the surface of the lithium ion secondary battery 100, and the displacement of layers occurring at the bent portions causes the electrode disposed on the inner side of the curve to be sagged, thereby varying the distance between the electrodes here. Therefore, the distance between the electrodes is not uniform inside the lithium ion secondary battery 100, and gaps occur between the respective electrodes and the separator. This causes a problem of deterioration of battery performance, such as the lowering of capacity retention characteristics.
As a lithium ion secondary battery in the form of a sheet as described above, there is proposed a battery in which the respective electrode sheets and the separator are tightly joined by bonding with a solvent-type adhesive (see, for example, Patent Document 1).
According to the lithium ion secondary battery described in Patent Document 1, the electrode sheets and the separator are bonded through an adhesive layer, whereby the flexibility of the bent and curved lithium ion secondary battery are ensured, and the loosening or displacement of the mutual contact between the electrode sheets and the separator can be suppressed.